The last four years of this program were devoted toward three principal objectives: (a) to examine the structure-activity relationships for inhibition of N-nitrosomethylbenzylamine (NMBA)-induced tumorigenesis in the rat esophagus; (b) to identify the P450 enzyme(s) responsible for the metabolism of NMBA in the rat esophagus, and to determine the effects of isothiocyanates on its/their activity(s); and (c) to identify inhibitors of tumor promotion/progression in the rat esophagus. With respect to the first objective, 3-phenylpropyl isothiocyanate (PPITC) was found to be a more effective inhibitor of NMBA-esophageal tumorigenesis than phenethyl isothiocyanate (PEITC) reported previously. However, the longer chain isothiocyanate (PHITC) actually enhanced NMBA-esophageal tumorigenesis. With respect to the second objective, our current data indicate that one or more cytochrome P450 enzymes of the 2A subfamily are involved in the metabolism of NMBA in the rat esophagus. To determine the precise mechanism of actions of PEITC and PPITC, it is necessary to know which P450(s) are responsible for NMBA metabolism. With respect to the third objective, PEITC, calcium, sulindac and ellagic acid were evaluated for their ability to inhibit tumor promotion/progression in the rat esophagus; all compounds were found to be inactive, with the exception of the high concentration of ellagic acid. An important goal of our research is to identify effective inhibitors of tumor promotion/progression in the esophagus, for these inhibitors could be useful in the prevention of esophageal tumor development in humans. Our recent finding that PPITC is an exceptional inhibitor of NNN-induced tumorigenesis provides a basis for the further study of the inhibitory effects of PPITC and related compounds on NNN-induced esophageal tumorigenesis as well as a basis for a toxicity study of PPITC. Therefore, our specific aims are: (1) To confirm that CYP2A3 or a closely related CYP2A enzyme is present in the rat esophagus and that his P450 efficiently metabolizes NMBA; (2) To further identify inhibitors of tumor promotion/progression in the rat esophagus; (3) To evaluate combinations of inhibitors of tumor initiation (i.e., PEITC) and of promotion/progression for their relative efficacy in inhibiting rat esophageal tumorigenesis; (4) To compare the chemopreventative efficacies of PEITC, PPITC, and their N-acetylcysteine conjugates on NNN-induced esophageal tumorigenesis using various biomarkers as endpoints; and (5) To evaluate the effects of PPITC in a 13-week toxicity study conducted in F344 rats.